The present invention relates to a moisture sensitive element for measuring a humidity or dew point and a method of manufacturing the same.
Conventionally, moisture in an atmosphere has been regarded as a very important factor having influences on quality. It is being required to measure and control an absolute moisture amount in a variety of fields, e.g., humidity control in a room, moisture control in heat treatments such as carburizing and tempering, humidity adjustment of blast furnace ventilation air, and humidity measurement and adjustment of closed vessels in a warehouse, a storehouse, and a laboratory. Recently, humidity control in semiconductor fabrication factories has become essential, and it is being increasingly demanded to use this technology in the growth control of agricultural products. On the other hand, dew-point measurement in weather conditions is being increasingly required for purposes more closely related to life, e.g., a laundry forecast, as well as a weather forecast. To meet these demands, several absolute moisture amount measuring methods have become popular.
As one method of measuring an absolute moisture amount, a lithium chloride (LiCl) dew-point meter using the deliquescence of lithium chloride measures a dew-point temperature from a change in the electric conductivity of lithium chloride with moisture absorption and a change in the vapor pressure of an aqueous lithium chloride solution with a temperature change. A moisture sensitive element of this meter is manufactured by impregnating a glass fiber tape with an aqueous solution of lithium chloride and spirally winding two parallel thin metal wires as electrodes on the resultant tape. When an AC voltage is applied to the two electrode wires, a current is generated between the electrodes to raise the solution temperature. Simultaneously, moisture in the aqueous solution evaporates to saturate the solution, so the crystal of lithium chloride starts precipitating. When the crystal starts precipitating, the electric resistance of the aqueous solution abruptly increases to reduce the current, and this suppresses the temperature rise. Consequently, a temperature corresponding to the ambient water vapor pressure is held. If the ambient water vapor pressure decreases, the temperature further lowers because the moisture in the aqueous solution evaporates to precipitate the crystal. If the ambient water vapor pressure increases, on the other hand, the moisture is absorbed to break the saturated state, the current increases, and the temperature rises. In this manner the temperature of the aqueous lithium chloride saturated solution is so held as to equilibrate with the ambient water vapor pressure. If this equilibrium temperature is known, the dew-point temperature can be calculated.
If, however, the moisture sensitive element of this lithium chloride dew-point meter is left to stand for long time periods in particularly a high-temperature, high-humidity atmosphere, lithium chloride is rapidly eluted to significantly deteriorate the characteristics. Therefore, it is necessary to perform maintenance and management while periodically replenishing lithium chloride.
As described above, the following state results if the element of the conventional lithium chloride dew-point meter is left to stand for a long time in a high-temperature, high-humidity atmosphere. That is, lithium chloride having strong deliquescence absorbs water vapor in the atmosphere and dissolves in the absorbed water vapor to form an aqueous solution. Usually, a current is supplied to raise the solution temperature to evaporate the moisture. However, if the temperature and humidity of the atmosphere are too high, this moisture evaporation becomes unsatisfactory, and the concentration of the aqueous lithium chloride solution decreases. Consequently, the aqueous lithium chloride solution excessively increases its flowability and flows out from the glass fiber tape holding the solution. When the aqueous lithium chloride solution thus flows out, the amount of lithium chloride used in the moisture sensitive element reduces, and this increases the resistance for the same moisture amount. Accordingly, accurate humidity measurement can no longer be performed.
Conventionally, therefore, maintenance and management must be so performed that the lithium chloride concentration is held at a predetermined concentration by, e.g., periodically replenishing lithium chloride. This makes the management troublesome.